This invention is directed to an improved deposition method for applying thin, transparent metal oxide films onto glass substrates. In particular, the invention contemplates the deposition of transparent conductive metal oxide coatings of uniform thickness on glass substrates wherein the electrical resistivities of the coatings can be carefully controlled over a wide range. In the preferred embodiment of the inventive method, antimony-doped tin oxide films of uniform thickness having optical transmissions over 80% and electrical resistivities ranging between about 7-1000 ohms/square can be prepared.
Metal oxide films produced through the pyrolysis of a wide variety of metal salts are well known to the art. Suitable materials and mixtures for forming electrically conducting metal oxide films include the chlorides, bromides, iodides, sulfates, nitrates, oxalates, and acetates of tin, indium, and cadmium, and various combinations of tin and antimony, tin and indium, and tin and cadmium. Optionally, a similar hydrolyzable salt or other compound of a modifying metal such as zinc, iron, copper, or chromium may also be included. For a fuller discussion of such films, their formation, and their characteristics, reference is made to U.S. Pat. Nos. 2,546,706 and 2,546,707.
Basically, the conventional process for preparing metal oxide films through pyrolysis has contemplated heating a substrate, such as a sheet of glass, to a temperature commonly within the range of about 400.degree.-700.degree. C., and then exposing a surface of the heated substrate to a metal salt which decomposes under the influence of the heated surface to form a continuous adherent metal oxide film thereon. Customarily, the filming material is sprayed onto the heated surface in the form of vapors or an atomized solution carried by a stream of air.
The process is quite dependent upon physical timing and individual technique. More importantly, the thickness of the deposited film is not uniform. Furthermore, where very thin glass sheet comprises the substrate, thermal gradients resulting from the spraying causes warpage thereof, thereby exaggerating thickness variations in the deposited film.
Applications such as heater panels and common resistors do not demand critical control of deposition thickness, but with the development of gas discharge, electrochromic, electroluminescent, and liquid crystal display devices, there is the need for thin, transparent conductive films of exceedingly fine uniformity of thickness. For flat display systems, uniformity of film thickness is vital. Hence, under optimum conditions those films demonstrate essentially no light absorption loss, but scattering and reflection losses can be significant due to nonuniformity of thickness.
Therefore, the primary objective of the instant invention is to develop a method for preparing thin, transparent conductive metal oxide films of exacting uniform thickness through pyrolysis of metal salts. More particularly, it is an objective of the subject invention to produce, by pyrolysis, antimony-doped, tin oxide films of uniform thickness within the range of about 1000-3000 .ANG. exhibiting optical transmissions greater than 80% and electrical resistivities varying from about 7-1000 ohms/square.